Known electrostatic clamps that are used for substrate holders are usually provided with means for using a backfill gas, which is used to fill the spaces formed between protrusions located on the holder, a base plate that forms a ground layer for supporting the protrusions, and a backside of a substrate clamped by the substrate holder. Such a backfill gas enhances the heat transfer capability from the substrate to the substrate holder. In order not to spill this backfill gas in vacuum operating conditions, the clamping pressure should be larger than the backfill gas pressure, and in order to provide a leak-tight confinement, a wall is present having a contour that generally follows the contour of the substrate and which provides, when the substrate contacts the substrate holder, a gas-tight environment.
European patent application EP0947884 describes a lithographic apparatus having a substrate holder in which protrusions are arranged to improve the flatness of the substrate. These protrusions have a general diameter of 0.5 mm and are located generally at a distance of 3 mm away from each other and thereby form a bed of supporting members that support the substrate. A typical height of the protrusions is 5 μm. However, for electrostatic clamping configurations, especially for electrostatic clamping configurations where a hard rim is present for confining a backfill gas to the back of a substrate to be irradiated, the substrate tends to be supported unevenly, especially near the boundary of the protrusion configuration, due to the termination of support near the boundary edge. This may cause, depending on the degree of overhang, an upward lifting of the wafer or a downward “sagging” near the edges, which may result in unacceptable imaging quality.